High frequency wave transmission system



March 25, 1941.- F. 'A.KOLSTER 2,236,102

HIGH FREQUENCY WAVE TRANSMISSION SYSTEM Filed April 28, 1938 2Sheets-Sheet 1 I sou/Fa: 0F 047/?4-5/40 sou/m? 0F ULTRA- WAVES A/VQDl/FL-CT 2 SHORT WAVES I CU/F/PEWT con/M04 VOLT/1658 RECEIVING APPARATUSINVENTO R FEEDER/CK A. K0LSTR ATTORNEY 194-1 F. A. KOLSTER 2 HIGHFREQUENCY WAVE TRANSMISSION SYSTEM Filed April 28, 1938 2 Sheets-Sheet 2ATTORNEY Patented Mar. 25, 1941 UNITED ST PATENT OFFICE HIGH FREQUENCY wVE TRANSMISSION A STEM Application April 28, 1938, Serial No. 204,735

8 Claims.

The present invention relates to wave blocking devices and totransmission line arrangements incorporating such devices and adapted.for feeding waves to a restricted portion of a conductive structurewhile preventing the waves from passing out over the remainder of theconductive structure. Reciprocally the invention likewise relates totransmission line arrangements incorporating such devices and adaptedfor receiving waves collected by a restricted portion of a conductivestructure while suppressing the reception of waves collected by otherportions of the conductive structure. More particularly the inventionrelates to dipole antenna structures and means for feeding the same, orfor leading energy away from the same, Without distorting the radiationpattern so as to render it unsymmetrical.

It is an object of the invention to provide a wave suppressor unit whichcan be conveniently applied to a transmission line and convenientlyadjusted independently in its effective position and in its eifectivefrequency of blocking. Especially it is an object to provide such a waveblocking arrangement which can be applied to a continuous conductor atany intermediate point without interrupting the conductor, and which canbe adjusted longitudinally along the conductor and can independently betuned to various 1 frequencies.

It is a further object of the invention to provide a dipole antennastructure together with a lead-in line for the same which shall producea perfectly symmetric radiation pattern undis- 35 torted by the lead-inarrangement used for feeding energy to the dipole antenna-or leadingenergy away from the dipole antenna in the case of reception. Moreparticularly it is an object to provide such a dipole and lead-instructure whose radiation pattern shall be symmetric with respect to theaxis of the dipole, being defined by a surface of revolution whose axiscoincides with the axis of the dipole.

' It is a further object to provide such a dipole and lead-in"arrangement for producing such a pattern symmetric about the dipole axisand accurately adjustable with respect to the shape and orientation of asection taken in a plane passing through the dipole axis. In particularit is an object to provide a vertical dipole and lead-in arrangementwhose horizontal patterns of radiant action, 1. e. the sections in allhorizontal planes, shall all be undistorted circles, and whose verticalpatterns, 1. e. the sections in all vertical planes through the axis,shall all be alike and shall be adjustable in shape and inclination.Especially it is an object to provide such an arrangement in which theshape and inclination of such vertical patterns may be readily adjustedby adjusting the eifective length and height of the dipole.

It is a further object of my invention to provide such an arrangement inwhich one end of the dipole is defined by means of a wave blockingdevice applied to an otherwise continuous conducting structure, and inwhich the position of said wave blocking device may be adjusted to varythe length and effective position of the dipole, and in which the waveblocking device may be independently tuned to anti-resonance at adesired frequency.

Vertically disposed dipoles of suitable length and elevation above theearth are commonly employed for producing radiation patterns symmetricabout the dipole axis. Such patterns are required in certainapplications, particularly in course beacons for guiding airplanes alonga desired course or route. It has been suggested that in order toobviate the distorting influence of the lead-in line which feeds such adipole antenna, the two lead-in wires should be shielded and should becarried up through the lower half of the dipole antenna itself, which ismade hollow for this purpose. In such structures for the purpose ofpreventing the energy of the dipole from passing capacitatively into thelead-in line or its shield, and thus distorting the radiation pattern,it has been suggested to interpose specially designed bi-filar balancedchokes between the dipole and the lead-in wires, but such arrangementsare somewhat complex and are attended with certain difficultiesespecially at ultra-high frequencies.

It has also been suggested heretofore to feed the dipole by means of acoaxial line which extends up through a tubular lower limb of the dipoleand to space the outer conductor of this coaxial line accurately withrespect to the tubular lower limb of the dipole so that said outerconductor is out of contact and coaxial with the lower limb of thedipole. In such a system the outer conductor of the coaxial line isconnected to the tubular lower limb of the dipole only at the centralpoint of the dipole, and the lower limb of the dipole is designed to beof exactly such length that it will resonate with the correspondingportion of the outer sheath of the leadin line, as a quarter-wave lineso as to block the transmission of the high voltage and high currentwaves of the dipole over the outer conductor of the coaxial lead-inline. Such an arrangement, however, not only necessitates the accuratedesign of the tubular lower limb of the dipole so as to constituteexactly a quarter-wave line with precise corrections for the unusual endefiects, but also is inherently limited in its wave blocking action bythe reactance-to-resistance ratio or Q factor of the line constituted bythe tubular lower limb of the dipole and the corresponding section ofthe outer sheath of the coaxial feed line. Furthermore, such a systeminherently restricts the freedom of adjustment of the length of thedipole so that the length of the dipole cannot be freely varied toproduce the desired inclination or distortion, in respect of the shapeof the axial sections, of the toroidal radiation pattern. With such asystem neither limb of the dipole can be varied-to produce the desiredadjustments of the radiation pattern since the length of the lower limbis inherently fixed by the requirement that this lower limb itself mustresonate as a quarter-wave line to suppress the passage of theoscillatory energy of the dipole into the lead line, while the length ofthe upper limb is fixed by the requirement of balance between the twodipole limbs.

In accordance with the present invention, the disadvantages of both theabove mentioned systems are avoided and, at the same time, a very simpleand structurally convenient design is at 'tained. In accordance with myinvention, these advantages are attained by providing a special waveblocking device so designed that it can be attached to a unitarystructure, such as a conducting tube which may form the outer conductorof the lead-in line and may also constitute the lower arm of the dipoleantenna, without the need of cutting or otherwise interrupting thiscontinuous conducting structure. Because of the fact that this waveblocking device may be adjustably moved along the continuous conductorwhich forms both the outer conductor of the lead-in line and the lowerarm of the dipole antenna, the length of the lower arm of the dipoleantenna may readily be adjusted without providing any telescopic membersor other cumbersome and complex arrangements for the purpose. The waveblocking device of my invention is also independently adjustable to tuneit to exact antiresonance at the desired frequency, so that it is notnecessary to pre-design the apparatus exactly for a given frequency, butrather the apparatus can be approximately designed and then adjusted.

The wave blocking device of my invention is also useful in othercombinations and in itself,

' antenna conductors to produce a two-course beacon for guiding aircraftalong a desired route;

Fig. .3 represents a lead-in arrangement for separately feeding aplurality of dipoles in an array;

Fig. 4 is an elevation partly in section of the wave blocking devicewhich is incorporated in the systems of Figs. 1 and 2;

Figs. 5, 6, '7 and 8 are partially sectioned elevations of further formsof wave blocking devices which may be used in the systems of Figs. 1 and2.

Fig. 9 illustrates a form of wave blocking deduced by other antennae orby parasitic reflectors. The dipole l is supplied with ultra-short wavesfrom the source 2 over the coaxial line 3, the sheath of this coaxialline forming the lower limb of the dipole and the central conductor ofthe coaxial line being extended to form the upper limb of the dipole. Atelescopic extension 4 is provided at the end of the upper limb of thedipole for adjusting the length of this limb. The effective electricallength of the lower limb of the dipole is determined by the position ofa wave blocking device 5 which is slipped over the sheath of the coaxialline 3 and clamped thereto, and 1 which serves as hereinafter explained,to block the passage of Waves traveling down over the sheath of thecoaxial line. As more fully explained hereafter, the cap, or upperportion 6 of the wave blocking device 5 is electrically a part of thelower limb of the dipole so that this lower limb may be regarded asextending from the center of the dipole to the lower edge of the cap 6,thus having the length B as shown in the figure. The lower portion 1 ofthe blocking device 5 is electrically at the same potential as the lowerhalf of the coaxial line 3, which is preferably at ground potential. Theupper limb of the dipole has a length A as shown in Fig. 1. Because ofthe fact that the lower limb of the dipole is larger in diameter thanthe upper limb, and has at its end the extended capacity surface of thecap 6, the lengths A and B should be slightly different in order toprovide the same equivalent electrical length for the two halves of thedipole I. If desired, however, the upper limb of the dipole may beincreased in size so as to be similar to the lower limb, and in caseexact symmetry is essential the upper extremity of the dipole may beprovided with a cap similar the coaxial line 3 are in push-pullrelationship or phase opposition, and since the potentials atcorresponding points of the inner and outer conductors are equal andopposite, the waves traveling upward through the coaxial line 3 willhave substantially no external effects and will pass unblockedthroughthe device 5 which is located entirely outside of the coaxialline 3, as previously mentioned. On arrival at the center of the dipoleI, the waves will spread upwardly along the upper half of the dipole anddownwardly over the lower limb of the dipole, just as in the case ofwaves supplied to an ordinary dipole from a transmission line connectedto the dipole from one side. The upward waves along the upper limb ofthe dipole will be partly radiated and partly re flected from the opentermination of the dipole. Similarly the downwardly traveling waves onthe lower limb of the dipole will be partly radiated and partlyreflected, the reflection in' this case occurring at the lower edge ofthe cap 6 which effectively acts as an open termination aslaterexplained.

It will thus be seen that inspite of the fact that the lower limb of thedipole is integral with the sheath of the coaxial lead-in line, thewavedistribution on the dipole is generally similar to the wave distributionon an ordinary dipole fed by a lead-in line Which joins the dipole fromone side. Thus the design of the apparatus with respect to impedancematching and with respect to the standing wave pattern may under mostordinary conditions be made in accordance with the accepted theory ofdipoles fed by lead-in lines separately connected from the side, whileat the same time the distortions of the radiation pattern which would beintroduced by the presence of a lead-in wire asymmetrically positionedwith respect to the dipole, are completely eliminated. At the same timethe dimensional values and impedance relationship required for impedancematching or for producing a given standing wave distribution may be ingeneral determined in accordance with the known theory of separately feddipoles.

Under one particular condition, however, the arrangement of Fig. 1 willdiffer in respect to the standing wave distribution and impedancerelationship from an equivalent dipole separately fed by a transmissionline connected to the dipole from one side. Such a condition ariseseither when an unbalanced wave containing a longitudinal or cophasalcomponent is supplied to line 3 for feeding the dipole or when the twoarms of the dipole are for some reason made of unequal electricallengths so that the reflected waves returning to the center of thedipole do not arrive at the center of the dipole in phase opposition. Inthe latter case even if the wave supplied from the source were balancedan unbalanced wave would be produced by reflection, and if the lead-inarrangement were of the conventional type the unbalanced or longitudinalcomponent would tend to return cophasally over the lead-in line. In thearrangement of Fig. l, on the other hand, the passage of such anunbalanced wave over the lead-in line is blocked by the wave suppressorwhich, as later described, has practically the same eifect as an opencircuit with respect to waves traveling cophasally over the lead-in line3. Under the unbalanced condition, therefore, the system of Fig. 1 willbehave differently from an ordinary dipole since the unbalanced wavesrefiected from the dipole will be prevented from entering the lead-inline. Thus the detrimental effect of a slight unbalance of the wavesupply source or of the two halves of the dipole will be eliminated.

Expressed from another viewpoint, it may be said that the dipole willbehave in a similar manner to an ordinary dipole with respect to wavestraveling in push-pull fashion up or down the transmission line, so thatwith respect to such balanced or push-pull waves the feeding of thedipole and the matching of impedances may be effected in accordance withthe well-known theory. With respect to any unbalances, however, whichtend to create a cophasal excitation of both halves of the dipole, or inother words, which tend to cause the dipole to oscillate as a singlecapacity antenna instead of as a dipole, the arrangement of the presentinvention eliminates any possibility of such excitation of the dipole.This is in many cases a very valuable feature, since a slight cophasalexcitation of both halves of the dipole so as to radiate as a singlecapacity antenna tends to superpose upon the desired radiation patternan undesired radiation pattern which sharply distorts the radiation.

Thus it is evident that a dipole fed in accordance with my invention notonly is free from the distortions due to a lead-in line disposedalongside of the dipole so as to distort the field of the latter, butalso is inherently free from longitudinal waves or waves which are notin true phase opposition, even if the source or the two halves of thedipole are slightly unbalanced.

Fig. 4 is an elevation partially in section which more clearly shows theexact construction of the wave blocking device 5. Referring moreparticularly to Fig. 4, it will be seen that the wave blocking device 5consists essentially of the cap 6 and the body portion 1 as generallyindicated in Fig. 1. Both the cap and the body portion are provided withscrews 8 which serve to clamp these portions to the sheath of thecoaxial line 3, and the two portions 6 and l are shaped so that togetherthey define with the sheath of the line 3 an annular or roughly toroidalcavity 9. The body por tion I is provided with a flange l0 which isnormally closely adjacent to the cap 6 so as to form a capacitytherewith. By adjusting the position of the body portion 1 along theline 3 the value of this capacity may be adjusted so that .this capacitytogether with the inductance of the toroidal cavity 9 forms ananti-resonant wave suppressor circuit tuned to the frequency of thewaves to be blocked. In case it is desired to employ the wave blockingdevice 5 in the open, the cap 6 is preferably provided with a downturned rim to prevent the entrance of rain or snow into the cavity 9.Furthermore, suitable draining means may be provided at the bottom ofthe body portion I.

When the wave blocking device 5 is properly tuned to anti-resonance withrespect to any given frequency, it operates as a very efiicient wavesuppressor and almost completely blocks passage of longitudinal orcophasal waves over the line 3. With wave lengths of the order of twoand onehalf meters a suppression of approximately twenty decibels may beprovided by one single wave suppressor of convenient dimensionsconstructed as shown in Fig. 4. The dimensions of the wave suppressormay be very small in comparison with the wave length to be blocked. Theaxial length, which is preferably the largest dimension, should besubstantially less than onequarter wave length and preferably of theorder of one-tenth wave length or less. At the same time it is evidentthat since the wave blocking device is entirely external to the line 3,this device will not in any manner impede the passage of push-pull orbalanced waves which travel in phase opposition over the two conductorsof the coaxial line 3. Thus so far as the normal pushpull wavestransmitted over the line are concerned, the wave blocking device willhave no effect and the waves can be transmitted to and from the antennain the usual manner.

In case it is desirable to adjust the position of the wave blockingdevice without disturbing the adjustment of its tuning, the cap portion6 and the body portion 1 may be clamped to a further tube which fitsclosely over the outer sheath of the coaxial line. Either the bodyportion 1 or the cap portion 6 may be fixed with respect to this tube,leaving the other portion adjustable, but preferably that portion of thewave blocking device which lies toward the waves tobe suppressed shouldbe fixed with respect to the auxiliary tube so that adjustment of theother portion will not vary the apparent position of the wave blockingdevice even slightly. The auxiliary tube to which both the cap 6 andbody portion 1 are attached may then be adjustably clamped to the sheathof the coaxial line 3 so that the wave blocking device as a whole may beadjusted to any desired position.

The device shown in Fig. 4 and above described, is especially adaptedfor use in the position shown with the cap 6 at the top of the device soas to exclude moisture. In cases where the exclusion of the weather isnot important, the device of Fig. 4 may be mounted in any position, andin any case it may be used. to suppress longitudinal or unbalanced wavestraveling along the line 3 in either direction. In. some cases it ispreferable to arrange the device with the cap 6 lying toward the wavesto be suppressed since this cap ordinarily has less surface area than.the body portion 7 and therefore the waves to be suppressed will extendover a smaller area. In case it is desired for this reason or for anyother reason to mount the device with the cap downward, the constructionshown in. Fig. 5 may be advantageously used. The structure of Fig. 5 isgenerally similar to that of Fig. 4, but a downwardly depending rim isprovided on the body portion instead of on the cap in order to excludethe weather when the device is mounted with the body portion above thecap.

The structure shown in Fig. 6 is also similar in principle to thestructure of Figs. 4 and 5, but in this structure both portions haveapproximately equal surface areas, and the capacity surfaces arepositioned approximately at the center of the toroidal cavity 9, asclearly shown in Fig. 6.

The structure shown in Fig. 7 may be preferred in cases where it isdesired to reduce the diameter of the wave blocking device. Thisstructure has no radially extending flanges but rather the cap and bodyportions overlap one another axially so as to provide a capacity betweentwo concentric surfaces. In. case it is desired to arrange a dipole suchas shown in Fig. 1 with the upper limb of the dipole structurallyexactly like the lower limb, a wave blocking device of the type shown inFig. '7 may advantageously be used and then each limb of the dipole maybe built out or enlarged to the same diameter as the wave blockingdevice so that there will be no discontinuity in either limb of thedipole.

In some cases where a very high degree of suppression of the wavestraveling along the coaxial line 3 is required, it may be advantageousto provide two or more wave blocking devices such as 5 of Fig. l, incascade along the line. Such an arrangement not only provides a greaterattenuation of the wave to be blocked, but also provides further degreesof freedom so that the termination characteristics of the blockingarrangement may be adjusted independently of the degree of blocking andthe position of the devices. For example, by employing two wave blocldngdevices immediately adjacent one another in the system of Fig. 1, theupper one of these two devices may be adjusted to provide a desirabletermination characteristic or reflection coefiicient for the lower limbof the dipole, while the second wave blocking device located immediatelybelow the first one may be adjusted to more completely attenuate thedownwardly traveling Waves so as to keep them out of the lower portionof the coaxial line 3. In case it is desired to employ two wave blockingdevices immediately adjacent one another, the arrangement shown in Fig.8 may be advantageous. In this arrangement two wave blocking devices areconstructed as a single unit having a common cap portion 6 and two bodyportions '1 and 1'. All three portions of the wave blocking device ofFig. 8 may be directly clamped to the coaxial line 3 by screws 8 or maybe'clamped to an auxiliary tube which is then clamped over the coaxialline as described with respect to the structure of Fig. 4.

Fig. 3 schematically represents an arrangement for feeding two separatedipole antennae over two separate coaxial lines. Such an arrangement maybe desirable in directive arrays or may even be used for separatelytransmitting two different signals on two separate antennae with thesame carrier frequency. As clearly shown in Fig. 3 two dipole antennae Iand I are each supplied with ultra-short waves from the transmittingapparatus 2 over two separate coaxial lines 3 and 3'. Adjacent eachantenna a wave blocking device 5 or 5' is provided to suppress thepassage of waves down the coaxial line. In order to make up thecontinuous conducting sheaths 3 and 3' so as to avoid distortions oftheir radiant patterns by the presence of this long continuousconductor, an additional wave blocking device H is provided at anintermediate point between the antennae and the transmitting apparatus2. Such auxiliary wave blocking device I! may be arranged as shown toblock waves along both coaxial lines simultaneously. The details ofconstruction of the wave blocking device may be more clearly seen inFig, 9. As shown in Fig. 9 the wave blocking device I is essentiallysimilar to the wave blocking device 5 as shown in the other figures, andespecially is similar to the device shown in Fig. 4. The device H,however, is arranged to surround two separate coaxial lines 3 and 3' andprovides instead of a simple toroidal cavity 9, a figure 8 shaped cavity9. If the two coaxial lines 3 and 3 are arranged in very closeproximity, however, the cavity 9 may constitute substantially a toroidinstead of a figure 8. In such a case waves traveling in push-pullfashion over the two inner conductors of the two lines would not beblocked.

Fig. 2 represents a transmitting beacon for providing course indicationsfor aircraft. As shown in this figure, the dipole is fed over a coaxialline 3 from the wave source 2 and is terminated by the telescopic endsection 4 and the wave blocking device 5 just as described in connectionwith Fig. l. The rotationally symmetric radiation pattern of the dipoleI must be modified to produce a useful course indicating pattern, andfor this purpose the two reflecting dipoles 2| and 3| are provided atequal distances on each side of the principal dipole Each of the dipoles2| and 3| is provided with adjustable telescopic extensions at each endsimilar to extension 4 of dipole I, in order that the reflecting dipolesmay be tuned so as to produce. the desired radiation pattern. The tworeflecting dipoles 2| and 3| are provided with relay 22 and 32 forrendering the two reflecting dipoles alternately efiective according tothe desired keying code. For this purpose the relay 22 is arranged sothat its contacts are normally closed, thus connecting together the twohalves of dipole 2| to render this dipole effective, while on the otherhand, the relay 32 is arranged so that its contacts are normally open,thus rendering the dipole 3| ineffective. Lines 23 and 33 connect thetwo relays to the inner and outer conductors of the coaxial line 3 atthe center of dipole I as shown, choke being provided to prevent thefiow of high frequency over the lines 23 and 33. These chokes may, ifdesired, be omitted since the lines are essentially located at rightangles to the electric field component of dipole l, but it is preferredto provide such chokes to completely avoid the flow of high frequency.Further similar chokes may also be provided inside the housings of therelay devices 22 and 32. The wave source 2 is arranged so as to supplynot only ultra-short waves but also suitable coded direct currentcontrol voltages over the line 3 for actuating the relays 22 and 32.Preferably the relays are so arranged that in response to a simultaneousdirect current voltage applied to both relays over line 3 the relay 22will open its contacts to disable dipole 2| slightly before the relay 32closes its contacts to render reflector dipole 3| efiective. Similarlyupon the cessation of the direct current control voltage over coaxialline 3 relay 32 -should release so as to disable the reflector dipole 3|before relay 22 operates to render reflector dipole 2| effective.

The wave blocking device 5 operates in the same manner as the waveblocking device 5 shown in Fig. 1, and serves in exactly the same way toterminate the lower limb of the dipole I. The ultra-short waves forenergizing the dipole 1 from the source 2 pass upward in push-pull orphase opposition relationship over the line 3 so that these are notblocked by the device 5. Also the direct current control voltages arenot in any way affected by the presence of the wave blocking device 5.

Although the above described antenna systems incorporating my novel wavesuppressing device have been described and shown as transmitting antennaarrangements, it will be understood that my invention is also applicableto receiving antenna arrangements. An of the wave suppressing structuresshown in Figs. 4, 5, 6, 7, 8 or 9 may be employed Without change forsuppressing Waves in a receiving antenna arrangement instead of in atransmitting antenna arrangement. Furthermore, any of the antennasystems of Figs. 1, 2 or 3 may be used as receiving antennae bysubstituting a receiving equipment for the sources of waves 2 in Figs. 1and 2. In order to illustrate the applicability of the invention toreceiving antenna systems, the element 2 of Fig. 3 has been designatedas either a transmitting or receiving apparatus.

It should furthermore be understood that the wave suppressor unit of myinvention may also be used not only for blocking waves, but also forpartially blocking such waves or generally for controlling thetransmission and reflection thereof. The wave blocking unit of myinvention may likewise be used for suppressing waves in transmissionsystems not involving any antenna, as for example, in systems fortransmitting waves from point to point over a coaxial line. Also waveblocking units in accordance with my invention may be employed tocontrol the transmission of longitudinal waves not only in coaxial linesbut also in shielded lines having two or more conductors within onesheath.

Although I have shown and described certain particular embodiments of myinvention for the purposes of illustration, it will be understood thatmodifications, alterations and adaptations thereof occurring to oneskilled in the art may be made without departing from the scope of theinvention as defined in the appended claims.

What is claimed is:

1. A coaxial transmission line having a sheath,

a conductive shell around said sheath at an intermediate position alongsaid sheath, said shell being shaped to enclose an annular cavity aroundsaid sheath, and capacity means for tuning said shell to a desiredfrequency whereby the transmission along said line of waves of saidfrequency may be controlled.

2. A coaxial transmission line having a sheath, a discontinuity at oneposition in said line, a conductive shell around said line at a secondposition on said line and outside said sheath, said shell being soshaped as to enclose an annular cavity around said sheath, means forturning said shell to a frequency to be transmitted over said line, andmeans for varying the distance between said discontinuity and saidconductive shell by independent changing of said first and secondpositions on said line.

3. A coaxial transmission line having a sheath and an inner conductor,and a resonant device coupled to said sheath for controlling thetransmission along said line of waves of a certain frequency, whichcomprises a hollow shell shaped to enclose an annular cavity around saidline, said shell being conductive and forming about said cavity aone-turn toroidal conducting circuit and having a gap interrupting saidcircuit, and capacity means forming a lumped capacity across said gapwhereby said device is tuned to said certain frequency.

4. A multi-conductor transmission line having a sheath and a resonantdevice coupled. to said sheath for controlling the transmissionalong-said line of waves of a certain frequency which cedar prises ahollow shell shaped to enclose an annular cavity around said linesubstantially less than' one-quarter wavelength in axial length at saidfrequency, said shell being conductive and forming about said cavity aone-turn toroidal conducting circuit and having a gap interrupting saidcircuit, and capacity means forming a lumped capacity across said gapwhereby said device may be tuned to said certain frequency.

5. A coaxial transmission line and a resonant device according to claim2, said device being slidably positioned on said sheath whereby saiddevice may be adjusted in position along said line.

6. A radiant acting system for operation at a certain frequencycomprising a radiant action dipole having a hollow first limb and asecond limb, a multi-conductor transmission line ex tending to saiddipole, at least one conductor of said line passing through said hollowfirst limb and connected to said second limb and a wave blocking devicebelow said dipole comprising a shell enclosing an annular cavity aroundsaid line substantially less than a quarter wavelength in axial lengthat said frequency, and means for turning said shell to said frequencywhereby waves of said frequency may be blocked from passing over saidsheath.

7. An antenna system for operation at a certain frequency whichcomprises a coaxial transmission line having a sheath and an innerconductor, the inner conductor being extended beyond the end of thesheath to form a first dipole limb, a wave blocking device on saidsheath spaced a distance from the extended inner conductor comprising ashell enclosing an annular cavity around said sheath substantially lessthan a quarter wavelength in axial length at said frequency, and meansfor tuning said shell to said frequency, said device being so tuned asto substantially block the passage of waves flowing over the outside ofsaid sheath whereby the portionvof the sheath between the said deviceand the end of the sheath constitutes a second dipole limb.

8. A plurality of coaxial transmission lines each having a sheath, aconductive shell around said lines at an intermediate position alongsaid line, said shell being shaped to enclose an annular cavity aroundsaid sheaths, and adjustable capacity means for tuning said shell to adesired frequency whereby the transmission along said lines of waves ofsaid frequency may be controlled.

FREDERICK A. KOLSTER.

